(a) Field of the Invention
The invention relates to a DC to low frequency AC power conversion system. More specifically, the invention relates to such a system which uses two parallel high frequency link stages.
(b) Description of Prior Art
Prior art static power converters which incorporate a variable high frequency link stage are well known in the art. Such converters offer several advantages, such as simple commutation, reduced size of magnetic and reactive components, and fast response. Load commutated resonant converters with sinusoidal output voltage are usually employed to realize the high frequency link. The link frequency is typically of the order of a few kiloHertz. Low frequency (60 Hz) output power is obtained from the high frequency link through typical cycloconversion methods. See, for example, U.S. Pat. No. 3,742,336, June 26, 1973, B. D. Bedford, "Versatile cycloinverter power circuits"; P. M. Espelage and B. K. Bose, "High-frequency link power conversion", IEEE Trans. Ind. Appl., Vol. IA-13, pp. 388-394, September/October 1977; and L. Gyugyi and F. Cibulka, "The high frequency base converter--a new approach to static high-power conversion", IEEE Trans. Ind. Appl., Vol. IA-15, pp. 420-429, July/August 1979.
In using cycloconversion methods for power conversion, the input to the cycloconverter is typically of a constant amplitude. Under such conditions, the firing angle .alpha. of the cycloconverter has to be modulated about a quiescent value, using a sinusoidal reference signal, in order to achieve a sinusoidal output voltage. However, if the input to the cycloconverter were amplitude modulated, then the firing angle would not have to be modulated. In such a case, a sinusoidal output voltage could be constructed by simple rectification/inversion. This process could be viewed as a form of cycloconversion in which the modulation process is incorporated in the input voltage instead of the switching.
To accomplish such an input, one can employ, in accordance with the invention, two HF link circuits such that the difference between the operating frequencies of the two link circuits is twice the required output frequency. The difference between the two link voltages is a high freqency sinusoid enveloped within another sinusoid at the low output frequency.
It is of course known in the art to combine the outputs of two frequency generators or the like to obtain a difference frequency as is shown, for example, in U.S. Pat. No. 3,151,915, Oct. 6, 1964, Graybeal. However, this approach has not been used in static power converters, nor has it been used as an input to a cycloconverter.